Conventionally, a known thermal-type airflow measuring device may be equipped to an air intake passage of an engine. The airflow measuring device may be configured to draw a part of air, which flows through the air intake passage, and to cause heat transfer with the drawn air, thereby to measure a flow rate of air drawn into the engine. The conventional airflow measuring device may include a case and a sensor assembly. The case may form an inner passage to flow drawn air therethrough. The sensor assembly may include a sensor chip and a circuit board. The sensor chip may be in a plate shape and may be configured to cause heat transfer with air and to generate an electric signal according to transferred heat. The circuit board may be electrically connected with the sensor chip via a bonding wire.
The sensor assembly may be inserted into and affixed to an insertion hole, which may be formed in the case, such that the sensor chip may be exposed to the inner passage. The sensor assembly may include a wire holding case to hold the bonding wire. The outer periphery of the wire holding case may have a reference surface, which may be configured to make surface contact with the inner circumferential periphery of the insertion hole for positioning the wire holding case. According to the configuration of Patent Document 1, the wire holding case has a surface, which is on the opposite side of the reference surface, and the surface may be biased from a projection, which is formed on the inner circumferential periphery of the insertion hole. Thus, the sensor assembly may be affixed to the insertion hole.
The wire holding case may have a space for accommodating the bonding wire. Accordingly, the wire holding case may decrease in rigidity around the space. That is, the wire holding case may include a low rigidity portion, which is easily deformed. It is presumable to determine the reference surface without consideration of the low rigidity portion. In this case, the projection may apply load, when the wire holding case is mounted to the insertion hole, to bend the wire holding case. Consequently, distortion may occur in the bonding wire and the sensor chip. Thus, accuracy of flow detection may be impaired.